In recent years, since inkjet recording enables to form an image simply and less expensively, it has been applied to various printing fields such as photography, various types of printing, marking, and special printing such as color filters. Specifically, by employing inkjet recording apparatuses which eject and control minute ink droplets, inkjet inks whose range of color reproduction, durability, and ejection adaptability have been enhanced, and paper for inkjet printing use whose ink-absorbability, color forming properties of colorants and surface glossiness have also been markedly enhanced, it has become possible to provide an image of high quality comparable to conventional silver halide photography.
However, the inkjet image recording system which requires special paper for inkjet printing use causes problems such as restriction of recording media usable in the system, and a subsequent cost increase of the recording media.
On the other hand, in an office and in a light printing, it is growing increasingly the need of a system which enables to carry out a full color printing at high speed without receiving restriction of recording media (for example, a plain paper, a coated paper, and an art paper, a double-sided printing on a plain paper etc.). In an electrophotographic method, there exists high speed type which can prints more than hundred papers in A4 size per minute. However, so as to satisfy requirement such as an easy-to-use, cost, consumed power, and printing high quality on various recording medium, there still remain many technical problems and requests by users are not satisfied enough.
In an inkjet recording method, when image recording is performed by using aqueous ink on a recording medium having high ink absorption such as plain papers, there is a problem that feathering in which ink bleeds and spread into a recording medium, or and a rear side penetration in which ink penetrates to a rear side of the recording medium. When image recording is performed on a recording medium without having an ink absorption layer such as coated papers, there is a problem that droplets coalescence with each others causes liquid gathering, resulting in an image mottling, or insufficient drying and fixing of ink.
Recently, for a purpose of high speed recording, developed is an inkjet printing in which an image is formed by a single pass system by using line scan type inkjet head. When printing is carried out by applying an aqueous ink to this line printing method (single pass method), liquid gathering or insufficient fixing on a coated paper tends to be worse than those by a multi-pass printing method. Further, as the single pass method requires faster drying, the single pass method requires large energy for drying and results in problem of high power consumption.
As one of the methods to resolve above problems, proposed is a method for forming an image recording by inkjet recording method onto various recording medium other than exclusive paper. Specific example include an ultraviolet ray curable inkjet recording method in which a photo curable compound such as an ultraviolet curable composition is cross-linked and cured by irradiating an actinic radiation such as ultraviolet (UV) ray after printing, or a phase change type inkjet recording method using wax ink being solid at room temperature.
Specifically, disclosed is a technology in which an inkjet ink comprising organic solvent, colorant and ultraviolet curable composition are printed onto polyethylene terephtalate film or vinylchloride sheet, and cured by irradiating of ultraviolet ray via ultraviolet ray irradiation apparatus, resulting in enhancing an ink fixation property by finger-touch dryness evaluation and solvent resistance by methanol-rubbing evaluation (For example, referred to Patent Document 1).
Constitution above enables to print with good fixing property on various recording medium. However, in the ultraviolet ray curing method, time lags occur between an ink deposition on the recording medium and an ultraviolet ray irradiation. During the time lags, ink wets and spreads and results in problem such that recorded dot size depends on a length of time lags, specifically on a constitution of apparatus or a recording speed. Further, when these constitutions are applied to a nonabsorbable recording medium such as a coated printing paper, because wet and spread state of ink changes according to a species of the recording medium, the recorded dot size changes according to the species of the recording medium, resulting in problem in which obtained image quality depends on the species of the recording medium.
Further, when an actinic energy radiation curing system is applied to a single pass method, problem occurs in which image quality tends to deteriorated by liquid gathering because deposited droplets tend to coalesce with each others before being irradiated by actinic energy radiation.
On the other hand, known is a method in which a solid ink at room temperature comprising wax having low melting temperature is heated (for example, to 120° C. or more), melted, ejected in melted state and solidified immediately after depositing on a recording medium. By applying this method, moderate image quality can be obtained even on a recording medium having ink absorption such as plain paper. However, in order to enhance friction resistance of the obtained image, it is necessary to enhance a hardness of an ink after solidification, and consequently wax having high melting temperature is needed so as to have a desired hardness. For example, in case of using wax having melting temperature of 120° C. or more, heating ink at high temperature as 120° C. or more becomes load on heat resistance of recording head and constituted materials of ink supply system in an inkjet printing apparatus, resulting in difficulty in keeping durability of the printing apparatus and having to use a large size apparatus. Specifically, in a method of using piezo element as a recording head, because it has Curie temperature as the upper temperature limit, there is natural limit to the upper limit for heating.
Further, disclosed is a method in which ink comprises alkyl aryl cycloalkanol as a gelling agent and the gelling agent ink functions to improve bleeding between color dots (for example, referred to Patent Document 2). However, by the method proposed in Patent Document 2, ink solidification function is not sufficient because only gelling agent is used to function as ink solidification. Specifically, in case of conveying plain papers at high speed in high speed printing, there occurs a problem that ink image is peeled off from the recording medium. Further, Patent Document 1 does not disclose relating to using an actinic radiation curable composition in combination with gelling agent in an ink at all.
Further, disclosed is a technology in which a bleed-through resistance and print quality is improved in case of printing on plain paper by adding oil gelling agent to an ultraviolet ray curable inkjet ink which has a function to turn into gel an organic solvent (for example, referred to Patent Document 3). However, there are some problems such that thixotropy appears in some kind of gelling agents or in case of storing in low temperature as 0° C., precipitation of compound occurs which is estimated as gelling agent    Patent Document 1: Unexamined Japanese Patent Application Publication (hereinafter referred to as JP-A) No. 2002-241654    Patent Document 2: JP-A No. 11-315245    Patent Document 3: JP-A No. 2005-126507